Bovine pneumonic pasteurellosis associated with Pasteurella haemolytica Al is a major cause of sickness and death in feedlot cattle. Although vaccination with this organism might be expected to produce immunity to the disease, experimental trials and field studies using conventional formalinized bacterins have failed to show a protective effect, in fact, vaccinated animals are frequently more suspceptible to the disease than their non-vaccinated counterparts.
Immunization with live P. haemolytica has been shown to protect cattle against experimental challenge exposure to the bacterium. P. haemolytica produces a cytotoxin with specificity for ruminant leukocytes. This may contribute to the pathogenesis of pneumonic pasteurellosis by impairing primary lung defense and subsequent immune response or by induction of inflammation as a consequence of leukocyte lysis.
The protective capability of cytotoxic supernate from P. haemolytica has been used as a vaccine. This preparation contains numerous soluble antigens from the bacterium which may be important in protection. These soluble antigens include a ruminant-leukocyte-specific cytotoxin, serotype-specific soluble surface antigens, neuraminidase and protease. An example of such vaccine and its development is disclosed in applicant's copending U.S. patent application Ser. No. 821,197 filed Jan. 22, 1986. Developments of vaccines from the crude cytotoxic supernate requires the purification and characterization of these antigens which can become a difficult and costly process.
Recent advances in molecular biology have provided a new approach in the characterization of bacterial determinants involved in pathogenicity. The particular genes which code for bacterial antigens can be isolated by molecular cloning using various recombinant DNA techniques. For example, it is known the genes which code for the heat-labile enterotoxins of E. coli of porcine and human origin have been isolated. The cholera toxin genes from Vibrio cholera have also been isolated using the cloned E. coli elt genes as a hybridization probe. Genes encoding the enzymatic moiety of the exotoxin A from Pseudomonas aeruginosa and the phospholipase C determinant of P. aeruginosa have also been successfully cloned and expressed in E. coli. These cloned genes greatly facilitate the analyses of the pathogenic and immunogenic characteristics of the toxin protein products as well as enabling the characterization of the genetic organization and regulation of expression of these bacterial toxins. Hence, an alternative method involving the isolating of genes coding for the soluble antigens of P. haemolytica would be an important advance over existing vaccine developed from culture supernatant.